Divalent manganese in reduced LaMnO 3− δ—effect of oxygen nonstoichiometry on structural and magnetic properties

Abstract

By means of topotactic low temperature reactions in a narrow temperature window, trivalent manganese in the LaMnO 3 perovskite is partly reduced to divalent, i.e., LaMnO 3− δ . Presently, phase relations, crystal structure and magnetic properties are described for metastable samples with nominal composition LaMnO 3− δ (0.00⩽ δ<0.25). A Jahn–Teller deformed orthorhombic structure ( Pnma) prevails for 0.00⩽ δ⩽0.08. A two phase region separates it from a second solid solution region with regular GdFeO 3-type orthorhombic structure, 0.20⩽ δ<0.25. The crystal structure of LaMnO 2.80 was determined by combined refinements of high-resolution powder synchrotron X-ray and neutron diffraction data; space group Pnma, a=555.23(1), b=797.16(2), c=557.41(1) at 298 K. The cooperative Jahn–Teller distortion is lost when the average manganese valence is outside 3.00±0.15. For 0.00⩽ δ⩽0.20 antiferromagnetic order occurs below T N≈140 K. For LaMnO 2.80 the A x -type antiferromagnetically ordered moment is μ Ax =1.5(2) μ B at 8 K. The low value is probably a result of spin frustrations as earlier seen for oxidized LaMnO 3.08 with the same crystal structure. The present study of extending the valence range of normal perovskites into the mixed Mn II/Mn III regime opens for novel studies of manganites of interest for CMR purposes.